1. Field of the Invention
The present invention relates to a liquid crystal alignment polymer, more particularly to a polyarnic acid alignment layer material obtained by the polymerization of aromatic diamine with dianhydride, and able to form a liquid crystal alignment layer having a pretilt angle of below 2 degrees.
2. Description of the Prior Art
A liquid crystal display (LCD) plays an important role in recent displays because it is small in volume and light in weight, is easily portable, has low working voltages, power consumption and good properties, and can be produced in scale. Different pretilt angles are needed for applications in different types of liquid crystal displays. Therefore, it is necessary to develope different alignment layer materials in the electrooptical device market. Traditional alignment layers in a TN (twisted nematic) type liquid crystal display are tilted generally 2 to 4 degrees to the surface of the substrate so as to produce a liquid crystal display with low duty ratio. Recently, liquid crystal displays are being vigorously developed. It is particularly desirable to produce an alignment layer with much lower pretilt angle for use in a liquid crystal display with a high duty ratio. However, the lower the pretilt angle, the higher the working voltage and a greater slope of the voltage (V)-transmission (T) curve is necessary to improve the duty ratio and the contrast of the display.
The most important requirements for an alignment layer are: pretilt angle, coating and orientation which are directly aimed at the structure and finally the production of a liquid crystal display. The reason for having a pretilt angle is to prohibit the disclination loop of the liquid crystal display. If reverse-twist domains appear in the display, a disclination line is formed that causes the contrast to decrease. The quality of the coating will influence the evenness of layer thickness and the production of pin holes. If the alignment layer thickness is uneven or a pin hole is produced, the electrical current value becomes large and can even result in a short circuit. The orientation is caused by an interaction between the alignment layer and the liquid crystal material. When the alignment is not good, the display will have a defect because of the production of disclination loop.
The alignment layers are classified according to their uses and include a TN (twisted nematic) type liquid crystal display, a STN (super twisted nematic) type liquid crystal display, and a TFT (thin film transistor) type liquid crystal display. In order to meet the alignment requirement, dianhydride is normally used as a main component of a conventional alignment layer. However, the coating and scratch-resistance of the polymer formed from dianhydride generally can not achieve market requirements. Also several problems are created by its use, such as, poor yield and problems keeping a clean room in a liquid crystal factory full of powders.
In order to solve the above problems, EP 708149 and JP 06180448 disclose the addition of a siloxane-containing copolymer in alignment layer materials so as to increase the transparency and scratch-resistance of the resultant alignment layer. But the polarity of siloxane functional group is large and therefore the added copolymer causes problems with working voltages. JP 07287235 relates to the addition of linear structure in alignment layer materials. It has been found that this alignment layer can only be used in STN type liquid crystal displays since the pretilt angle of the formed alignment layer is increased. EP 683418 makes use of UV light to control the pretilt angle of the alignment layer by introducing some photosensitive resins in the alignment layer materials. To date, it is still not possible to obtain a very stable alignment layer and consequently there are problems with the liquid crystal display caused by use, time and temperature. In addition, EP 625722 discloses the use of a surface-treated silica to replace conventional alignment layers. A difficulty exists, however; mass production of silica is hard to achieve. JP 06180451 relates to the application of polyamideimide alignment layer in a chiral smetic liquid crystal display but such a liquid crystal display is not yet produced in scale.
Based on the above description, the present invention provides a new polyamic acid alignment layer material having a low pretilt angle, and the preparation thereof comprising adding a aromatic diamine functional group into conventional dianhydride and polymerizing the above mixture. The resultant polyarnic acid alignment layer obtained in the present invention possesses excellent coating and high adhesion.